Book holder



FIG. 1 is a perspective view of the book holder, with a book shown in broken lines for purposes of illustration only and forming no part of the claimed design;

FIG. 2 is a front view of the book holder;

FIG. 3 is a rear view of the book holder;

FIG. 4 is top view of the book holder;

FIG. 5 is a bottom view of the book holder;

FIG. 6 is a right side view of the book holder; and,

FIG. 7 is a left side view of the book holder. 

The ornamental design for a book holder, as shown and described. 